The maximum amount of information that can be transmitted over a classical noisy communication channel is known as Shannon capacity. Shannon's theorem states that, for a noisy channel, if a transmission rate is less than the Shannon capacity, there exist error correction codes that allow a probability of error at a receiver to be made arbitrarily small. Thus, given the right error correction codes, the transmission rate may approach the Shannon capacity. When a modulation alphabet of a communication channel are quantum states, a Holevo limit is an upper bound to the Shannon capacity of the communication channel coupled with any receiver. Achieving the Holevo limit for a communication channel may be quite challenging.